POLYMETALLIC MASSIVE SULFIDES AT THE MODERN SEA-FLOOR - A REVIEW

Polymetallic massive sulfides on the modern seafloor have been found i n diverse volcanic and tectonic settings at water depths ranging from about 3700 to 1500 m. These deposits are located at fast-, intermediat e-and slow-spreading mid-ocean ridges, on axial and off-axis volcanoes and seamounts, in sedimented rifts adjacent to continental margins an d in subduction-related backarc environments. High-temperature hydroth ermal activity and large accumulations of polymetallic sulfides, howev er, are known at fewer than 25 different sites. Several individual dep osits contain between 1 and 5 million tonnes of massive sulfide (e.g., Southern Explorer Ridge, East Pacific Rise 13 degrees N, TAG Hydrothe rmal Field) and only two deposits (Middle Valley and Atlantis II Deep, Red Sea) are known to contain considerably higher amounts of sulfides ranging between 50 and 100 million tonnes. This range (1-100 million tonnes) is similar to the size of many volcanic-associated massive sul fide deposits found on land. However, the vast majority of known sulfi de occurrences on the modem seafloor amount to less than a few thousan d tonnes and consist largely of scattered hydrothermal vents, mounds a nd individual chimney structures. Recovered samples from about 25 depo sits world-wide represent no more than a few hundred tonnes of materia l. The mineralogy of these samples includes both high (> 300 degrees-3 50 degrees C) and lower-temperature (< 300 degrees C) assemblages cons isting of varying proportions of pyrrhotite, pyrite/marcasite, sphaler ite/wurtzite, chalcopyrite, bornite, isocubanite, barite, anhydrite an d amorphous silica. Massive sulfide deposits in back-are environments additionally may contain abundant galena, Pb-As-Sb sulfosalts (includi ng jordanite, tennatite and tetrahedrite), realgar, orpiment and local ly native gold. Close to 1300 chemical analyses of these samples indic ate that the seafloor deposits contain important concentrations of Cu and Zn comparable to those of massive sulfide deposits on land. The se diment-hosted deposits, while being somewhat larger than deposits on t he sediment-starved mid-ocean ridges, appear to have lower concentrati ons and different proportions of the base metals due to fluid-sediment interaction. Initial sampling of sulfides in the back-are spreading c enters of the West and Southwest Pacific suggests that these deposits have higher average concentrations of Zn, Pb, As, Sb and Ba than depos its at the sediment-starved mid-ocean ridges. Gold and silver concentr ations are locally high in samples from a number of mid-ocean ridge de posits (up to 6.7 ppm Au and 1000 ppm Ag) and may reach concentrations of more than 50 ppm Au and 1.1 wt% Ag in massive sulfides from immatu re backarc rifts, that are dominated by felsic volcanic rocks. Preciou s metal contents of seafloor sulfides thus are well within the range o f those found in land-based deposits. Although massive sulfide deposit s have been found at water depths as shallow as 1500 m, boiling of the hydrothermal fluids may prevent the formation of typical polymetallic massive sulfides at shallower depths (i.e., less than a few hundred m eters), where the hydrostatic pressure is too low to prevent phase sep aration. In this case, mineralization with distinct epithermal charact eristics and significant amounts of precious metals can be expected.